The present invention pertains to deposition methods and systems. More particularly, the present invention relates to atomic layer deposition (ALD) methods and systems.
As integrated circuit (IC) dimensions shrink, the ability to deposit conformal thin films with excellent step coverage at low deposition temperatures is becoming increasingly important. Thin films are used, for example, in and/or for MOSFET gate dielectrics, DRAM capacitor dielectrics, adhesion promoting layers, diffusion barrier layers, electrode layers, seed layers, and/or for many other various functions. Low temperature processing is desired, for example, to better control certain reactions and to prevent degradation of previously deposited materials and their interfaces.
Conventional thin film deposition techniques, for example, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), are increasingly unable to meet the requirements of advanced thin films. PVD, such as sputtering, has been used for depositing conductive thin films at low cost and at relatively low substrate temperature. However, PVD is inherently a line-of-sight process, resulting in poor step coverage in high aspect ratio trenches and vias.
CVD processes can be tailored to provide conformal thin films with improved step coverage. Unfortunately, CVD processes often require high processing temperatures, resulting in incorporation of high impurity concentrations, and poor precursor utilization efficiency, leading to a high cost process.
Atomic layer deposition (ALD) has been used as an alternative to traditional CVD methods to deposit very thin films. ALD has several advantages over PVD and traditional CVD processes. For example, ALD can be performed at comparatively lower temperatures, has higher precursor utilization efficiency, and can provide conformal thin film layers, as well as provide control of film thickness on an atomic scale.
A typical ALD process differs significantly from traditional CVD processes. In a typical CVD process, for example, two or more reactant gases are mixed together in the deposition chamber, where either they react in the gas phase and deposit on the substrate surface, or they react on the substrate surface directly. Deposition by CVD occurs for a specified length of time, based on the desired thickness of the deposited film.
In an ALD process deposition cycle, for example, each reactant gas is introduced sequentially into a deposition chamber, so that no gas phase intermixing occurs. A monolayer of a first precursor is chemisorbed onto a surface on which material is to be deposited. Any excess of the first precursor is then pumped out, such as with the aid of an inert purging gas. Thereafter, a reactant is introduced to the deposition chamber to react with the chemisorbed species of the first precursor to form a monolayer or less of the desired material via a self-limiting surface reaction. The self-limiting surface reaction halts once the initially adsorbed precursor fully reacts with the reactant. Thereafter, excess reactant and any reaction by-products are pumped out, for example, with the aid of an inert purging gas. A desired film thickness is obtained by repeating the deposition cycle as necessary. The film thickness can be controlled on an atomic scale (e.g., angstrom scale) by controlling the number of deposition cycles.
Chemisorption occurs when adsorbed precursor molecules chemically react with active surface sites. Generally, chemisorption involves cleaving a weakly-bonded ligand (i.e., a portion of the precursor) from the precursor, leaving an unsatisfied bond available for reaction.
The ALD process temperature is carefully selected so that the precursor is sufficiently adsorbed (e.g., chemisorbed) on the surface on which the film is to be deposited. Further, the temperature must be such that the deposition reaction occurs with adequate growth rate and film purity. A temperature that is too high can result in desorption or decomposition, causing impurity incorporation. A temperature that is too low may result in incomplete chemisorption of the precursor, a slow or incomplete deposition reaction, no deposition reaction, or poor film quality (e.g., high resistivity, low density, poor adhesion, and/or high impurity content).
Such temperature selection is somewhat limiting on the ALD process. Plasma enhanced ALD, also referred to as radical enhanced atomic layer deposition (REALD), has been proposed to address temperature limitations of traditional thermal ALD. For example, in U.S. Pat. No. 5,916,365 to Sherman, entitled “Sequential chemical vapor deposition,” issued 29 Jun. 1999, the reactant for converting the adsorbed precursor passes through a radiofrequency (RF) glow discharge, or plasma, to disassociate the reactant and to form reactive radical species to drive deposition reactions at lower process temperatures. However, plasma-enhanced ALD still has several disadvantages. For example, plasma-enhanced ALD remains a thermal process similar to traditional ALD, since the substrate temperature provides the required activation energy, and therefore, the primary control for deposition reaction.
Further, for example, in U.S. Pat. No. 6,630,201 B2 to Chiang et al., entitled “Adsorption process for atomic layer deposition,” issued 7 Oct. 2003, an ALD process has been described which uses a plasma that includes energetic ions (e.g., argon+ions) and a plurality of reactive atoms to improve the plasma-enhanced ALD process. For example, as described therein, ions and atoms impinge on the surface of the substrate. Energetic ions transfer energy to the substrate, allowing reactive atoms to react with the chemisorbed precursor and to strip away unwanted ligands. The reactive atoms, in conjunction with energetic ions, act as the reactant to convert the adsorbed precursor on the surface to a monolayer or less of material resulting from the ALD cycle.
However, such low temperature ALD processing that uses plasma surface exposure and/or ion bombardment (e.g., ion bombardment using an ion source or ion gun) often results in at least some surface sputtering. For example, in one exemplary illustration, a capacitively coupled plasma may be generated above a wafer surface and provide ion bombardment with a mean ion energy of approximately 50 eV. For many materials, 35 eV is generally high enough to promote undesirable sputtering. Such sputtering is due to naturally-existing ion energy distributions that have a statistical fraction of ionic species with energies above the sputtering threshold. Such sputtering can result in low ALD film deposition rates in terms of fractions of angstroms per ALD cycle.